Threatwatch: Is Syria’s polio a danger to Europe?

Threatwatch is your early warning system for global dangers, from nuclear peril to deadly viral outbreaks. Debora MacKenzie highlights the threats to civilisation – and suggests solutions

The outbreak of polio in Syria may carry a major risk to Europe, epidemiologists in Germany warned on Friday. Paradoxically, this is because Europe has moved to using a safer vaccine that is crucial to the goal of global polio eradication.

Other epidemiologists argue, however, that Europe and other regions are at no greater risk than before, if a massive vaccination campaign kicking off now stops polio re-establishing itself across the Middle East. If that fails, though, there may be a double peril&colon; polio virus will be more widespread, and there may not be enough vaccine to contain it.

The situation is made all the worse because polio is spread in faeces, and sanitation, especially for the more than four million refugees within Syria, is overstrained and damaged by fighting.

Circulation problems

Martin Eichner of the University of Tübingen and Stefan Brockmann of the regional public health office in Reutlingen, both in Germany, warned in The Lancet that Europe is at risk from this outbreak.

In countries where polio virus either still circulates or there is a strong risk of its reintroduction, children get oral polio vaccine (OPV) containing a live, weakened virus. This replicates in the gut, inducing powerful immunity which both protects the child from disease and stops them spreading any polio virus they may encounter.

The problem with the oral vaccine is that in one in 2.7 million cases, the live virus can revert and cause paralysis – and can continue to circulate. For this reason, the Global Polio Eradication Initiative plans to switch the whole world to a killed version of the vaccine as polio disappears. Rich countries, where the risk of the vaccine exceeds the risk of encountering polio, have already done so. No country in the European Union has used the oral vaccine since 2010, some for much longer.

Killed vaccine, however, doesn’t induce the same powerful gut immunity. If children immunised with killed vaccine encounter polio, they won’t get sick, but they can acquire and spread the virus.

Ghost carriers

Most countries watch for a reintroduction of polio by testing any cases of paralysis that arise – but killed vaccine keeps infected people healthy. The German team calculates that polio might therefore spread for a year among people immunised with killed vaccine before giving itself away with a single case of paralysis. By then it might be widespread.

Nick Grassly of Imperial College London has mathematically modelled exactly this possibility. “We know the killed vaccine can reduce transmission enough to wipe out the virus in rich countries,” he says – Scandinavia and the Netherlands have used it to eliminate polio.

But in low-income populations where sanitation is poorer and transmission stronger, killed vaccine may not reduce spread enough to stop continuous circulation. In a paper published in the American Journal of Epidemiology in October, Grassly and his colleagues say that the virus might persist in such places, especially where high levels of vaccination prevent any visible cases.

Israel has now provided evidence of this. It has used only killed vaccine since 2005. It discovered wild polio virus in sewage near a low-income settlement last April, and continued to find it in sewage, and excreted by healthy youngsters, across the country until August. It is the first time polio has been found circulating without causing any disease.

The Israeli virus, the polio in Syria and polio found in Egyptian sewage last year are all closely related to strains circulating in Pakistan, where polio vaccination has been hampered by political opposition.

Risk to Europe

In late September the European Centre for Disease Control and Prevention (ECDC) in Stockholm assessed the risk to Europe posed by the Israeli virus and concluded that polio might invade unvaccinated populations in contact with Israel or other places where polio circulates, or with poor sanitary conditions, and then spread silently within nearby populations immunised only with killed vaccine. This also applies to the Syrian outbreak.

We might not know until it is widespread. The ECDC says only six EU countries still test sewage for polio, and surveillance for paralysis has become poor since the last polio outbreak in Europe in 1992.

Poor vaccination coverage in some countries might at least alert us to any such reintroduced polio, albeit at a high cost. In EU countries such as the UK and Austria, only 85 per cent of people are vaccinated, falling to 74 per cent in some boroughs of London.

That means tell-tale cases of paralysis would be more likely than they were in Israel, where 94 per cent of children are vaccinated. “But I’d like to see the UK, certainly London, do some environmental sampling for polio,” says Grassly.

Vaccination drive

Dina Pfeifer of the European office of the World Health Organization (WHO) in Copenhagen, Denmark, says Europe is not really at any more risk than before the Syrian outbreak, as long as that outbreak is contained quickly. The virus has always circulated in countries where it has never been eliminated – Nigeria, Afghanistan and Pakistan – and other places with renewed outbreaks, such as Somalia, she says.

Risk rises the more virus is circulating anywhere in the world, however, making rapid control in the Middle East vital.

If polio does reinvade anywhere else, live vaccine will be needed to stamp it out. There Europe has more problems&colon; only eight EU countries even permit the oral vaccine, says the ECDC, and none has a stockpile. Between the needs of the Middle East emergency, eradication efforts and routine vaccination around the globe, it isn’t clear if there is much more oral vaccine in the world left to buy if Europe suddenly finds it needs some.